Properties of a novel thermal sensitive polymer based
on poly(vinyl alcohol) and its layer-by-layer assembly
Hong Lu1,3, Anna Zheng1 and Huining Xiao2,3*1School of Materials Science and Engineering, Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science &
Technology, Shanghai 200237, China2State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China3Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3
Received 21 October 2006; Revised 12 December 2006; Accepted 12 December 2006
Structurally modified poly(vinyl alcohol) (PVA) was prepared as novel thermally sensitive polymers
by partially acetalyzing and/or ionizing the commercially available PVA. Their aqueous solutions
experience completely reversible polymer aggregation and dissolution above and below the lower
critical solution temperature (LCST), respectively. The LCST of a partially acetalyzed PVA (APVA)
can be readily controlled by the degree of acetalysis or the molecular weight of the starting PVA.
Introduction of a small amount of cationic group onto the APVA backbone increases the LCST
obviously, while the LCST is highly sensitive to NaCl concentration. ThenAPVA and cationic APVA
multilayers are assembled on rayon to make a thermal responsive fiber. The atomic force microscopy
(AFM) images of the surface reveal the increment of roughness stimulated by temperature.
Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: temperature responsive; poly(vinyl alcohol); self-assembly; atomic force microscopy (AFM); fibers
INTRODUCTION
Water-soluble thermal responsive polymers undergo fast,
reversible volume changes around their lower critical
solution temperature (LCST).1–6 Below LCST, the free
polymer chains are soluble in water and exist in an extended
random coil conformation that is fully hydrated. On the
contrary, above LCST, the chains hydrophobically fold as a
result of dehydration and assemble to form a phase
separating state. These polymers can be formed as gels,
particles, micelles, and capsules, etc. Therefore, they are
potentially very attractive for many engineering appli-
cations, such as tissues,7,8 controlled drug deliverer,9–12
and catalyst carrier,13–15 etc.
In this research, partially acetalyzed poly(vinyl alcohol)
(APVA) and cationic partially acetalyzed poly(vinyl alcohol)
(CAPVA) were used as novel thermal responsive polymers
with LCST because their hydrophilicity was reduced by the
reaction between PVA and acetaldehyde. It is widely known
that the poly(vinyl alcohol) (PVA) is a low toxicity16 and
relatively inexpensive polymer which has been produced on
a large scale commercially. In addition to the wide range
of industrial applications, PVA is also used in a variety of
biomedical applications, such as in the manufacture of
artificial kidney and pancreas,17–19 cell encapsulation,20–22
artificial articular cartilage,23 drug delivery,24–27 nerve
cuffs,28 etc. The minimized protein adsorption and biocom-
patibility of PVA are the main attractions for the popular use
of PVA as a biomaterial. APVA was first prepared by
Christova et al.29 from the reaction between PVA and
acetaldehyde in acid solution. However, there were very
limited publications, further investigating the properties of
APVA, and cationic-modified APVA has not yet been
reported. For CAPVA, not only the phase transition
temperature, strength, and sensitivity can be modulated,
but also ionic strength sensitivity is introduced by the
incorporating charges.
Cellulose is the most abundant natural biopolymer, and its
derivatives have many important applications in fiber,
paper, and paint industries. A variety of monomers were
grafted onto cellulose by different techniques to control
properties, such as hydrophobicity, adhesivity, selectivity,
drug delivery, wettability, and thermosensitivity.30–32 Com-
pared to grafting polymerization, the layer-by-layer (LbL)
method is a particularly simple approach to produce thin
films of controlled structure and chemical architecture for a
variety of applications, such as optical devices, membranes,
or biologically active coatings.33–35 In LbL, polymers
sequentially deposit on a substrate, where strong electro-
static attraction or hydrogen bonds occur between the
surface and polymers in the solution. The adsorption of
cationic polymers on cellulose fiber is practically possible
POLYMERS FOR ADVANCED TECHNOLOGIES
Polym. Adv. Technol. 2007; 18: 335–345
Published online 26 February 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/pat.891
*Correspondence to: H. Xiao, Department of Chemical Engineer-ing, University of New Brunswick, Fredericton, NB, Canada E3B5A3.E-mail: [email protected]
Copyright # 2007 John Wiley & Sons, Ltd.
because of its negatively charged surface. In this work, a
thermally sensitive fiber wasmade by assembling APVA and
CAPVA multilayers on a rayon fiber. Rayon is a cellulose
fiber with a smooth surface, compared to conventional
chemical wood fiber, because it is a manufactured fiber
composed of regenerated cellulose. The thermally sensitive
topography of a smooth surface, as observed by AFM, is
more reasonable and acceptable. The increment in roughness
or the reversible hydrophilic/hydrophobic properties of the
fiber surface, which is induced by temperature, potentially
benefits the strength development of fiber products aswell as
the retention of functional fillers or additives in conventional
or specialty papermaking processes. This is the first report
for the preparation of CAPVA and its application in
assembling of multilayers.
EXPERIMENTAL
MaterialsPVAs of three different molecular weights, 124,000–186,000,
61,000, and 13,000–23,000 g/mol, were obtained from
Aldrich. The degree of hydrolysis of PVAs was more than
98.0mol%. Acetaldehyde (FisherChemical) and glycidyltri-
methylammonium chloride (GTMAC; Fluka) were used as
received.
Synthesis of acetalyzed PVA (APVA)PVA solution was placed in a three-neck flask equipped with
a mechanical stirrer, dropping funnel and condenser. A
chosen amount of HCl was added and then acetaldehyde
was added dropwise at 158C for 30min. After the completion
of acetaldehyde feeding, themixturewas kept at 158C for 1 hr
and 308C for 24 hr. The reaction was stopped by neutraliz-
ation with 2 N NaOH aqueous solution. The solution
obtained was dialyzed for 48 hr with a dialysis membrane of
molecular weight cut-off 1000.
Synthesis of cationic APVA (CAPVA)PVA was cationized by GTMAC in a basic solution at 808Cfor 1 hr in a three-neck flask. The reactive mixture was
dialyzed for 24 hr after being neutralized by 1 N HCl
solution. Then it was acetalyzed and dialyzed following the
same procedure for synthesis of APVA.
Multilayer buildupAPVA and CAPVA (Mw¼ 124,000–128,000 g/mol) solutions
were prepared by dissolution in 5mM NaCl solution
to obtain 0.44 wt%. The silicon wafer with a 100 nm oxidized
layer and rayon fiber (from MiniFiber, Inc.) were used as the
substrates. The wafer was cleaned with 98% w/w H2SO4
solution at 608C for 4 hr. The substrate was alternately
immersed into the CAPVA solution and APVA solution for
10min each. Following each dip coating, the substrate was
rinsed thrice with D-D water and eight layers were
assembled on the substrates.
CharacterizationThe NMR spectra were recorded in DMSO-d6 or D2O at 258Cusing an Oxford 300MHz spectrometer operating at 300.13
and 75.5MHz for 1H and 13C nuclei, respectively. DMSO-d6
solutions were used to characterize the structure of APVA
andCAPVA. D2O solutions were used to calculate the degree
of acetalysis of APVA and degree of cationization of CAPVA.
Nondeuterated DMSO or H2O was used as an internal
reference. 1H, 13C, and DEPT (distortionless enhancement by
polarization transfer) experiments were performed at
standard pulse sequences. All samples were carefully dried
in a vacuum oven at 608C for 24 hr before the solutions were
made.
A HACH 2100AN turbidimeter was employed for the
LCST measurements. Samples were placed into a water bath
with temperature control. The samples were heated from 10
to 708C at a step of 2.58C. The samples were kept in the water
bath for 10min at each temperature, then taken out and dried
up quickly before inserting into the turbidimeter. The
turbidity was then recorded at each temperature.
A MultiMode Scanning Probe Microscope (Nanoscope
IIIa) with a J-scanner (maximum scan area is 125� 125mm2;
Veeco Instrument) was used to record the images of the PVA
film and was operated in contact and tapping mode. The tips
(Veeco NanoProbeTM; Digital Instruments) for contact and
tapping mode were NP-20 and TESP7, respectively. AFM
images were flattened by applying a first-order polynomial
fit to remove defects from the image, due to vertical (Z)
scanner drift.
Differential scanning calorimeter (DSC) experiments were
performed with a TA Instrument DSC Q100. All samples
were heated from 25 to 2458C at the rate of 108C/min and
held at 2458C for 5min, then cooled down to 258C at the same
rate and held at 258C for 5min. The samples were reheated
from 25 to 2458C at the rate of 108C/min again.
RESULTS
Molecular structure characterizationof APVA and CAPVAAPVA was synthesized when acetaldehyde reacts with the
adjacent hydroxyls on the PVA chain as shown in
Scheme 1a. Two kinds of acetal rings, meso and racemic,
shown in Scheme 2, resulted from isotactic and syndiotactic
PVA. Since not all of the hydroxyls reacted to form acetal
rings, APVA is essentially a copolymer of acetal ring and
vinyl alcohol. The microstructure of APVA is complicated by
the tremendous possibilities in constitutional and config-
urational variations of the copolymer.
CAPVA was synthesized by a two-step procedure. First,
the cationic monomer, GTMAC, was grafted on PVA
through the ring opening reaction of the epoxy group on
GTMAC with the hydroxyl group on PVA, as shown in
Scheme 1b. Then the cationic PVA was acetalyzed by
acetaldehyde to produce the CAPVA. NMR techniques were
applied in an attempt to assign the signals and to make the
quantitative analysis of the copolymer composition.1H spectra of APVA and CAPVA are shown in Fig. 1. As
for the symbols, A stands for APVA, the number after A is the
degree of acetalysis of PVA; C means cationization, the
number after C refers to the molar fraction of the cationic
group. Slanting letters are used to denote the carbons and
protons in the APVA and CAPVA. Resonance peaks of
methylene and methine protons a, e, e0, b, f, and f0 in the main
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
336 H. Lu, A. Zheng and H. Xiao
chains of A52 appear at 1.3–1.6 and 3.77 ppm.36–37 The
resonance at 4.7 and 4.9 ppm are assigned tomethine protons
g and g0 inmeso and racemic rings, respectively, because they
connect with two oxygens directly and their resonances shift
farther downfield. The biggest peak is assigned to the g in the
meso ring because the meso ring is less strained and
predominates over the racemic rings. The proportion of the
meso rings is around 85–90mol%, found by analyzing the1H signal integrals. Consequently, the peak at 1.0–1.2 ppm
was split into two peaks corresponding to themethyl carbons
d and d0 in the meso (downfield) and racemic (upfield) rings
too. The signals of the residual OH-groups are 4.44, 4.38, 4.28,
4.18 ppm.As for the 1H-NMR spectrum of CAPVA, the single
big peak at 3.15 ppm is assigned to the methyl proton m
because it is contributed by three isolated methyl groups and
neighbors the polar ammonium chloride group. The small
peak at 4.5 ppm belongs to hydroxyl proton k on the GTMAC
side group, which can be proved by 1H-NMR of cationic
PVA. The 1H-NMR spectrum of cationic PVA is not
presented here. The signals of the proton i, l, and j are
covered by the wide peaks of methylene and methine on the
backbone.
Since 13C spectra have a much larger chemical shift range
than the 1H spectra, they are expected to showmore resolved
signals and to provide more details on configurational
microstructures. The standard 13C-NMR spectra of APVA
and CAPVA are shown in Fig. 2. The methine carbons g and
g0 of the meso and racemic rings shift further downfield to
97.6 and 91.7 ppm, respectively, because of the same reasons
mentioned for 1H-NMR. The methine carbons f and f0 of themeso and racemic rings in the main chain are shifted
downfield to 71.5–73.5 and 67.0–69.0 ppm from PVA
methines since they are in the a-position to an OR group
instead of an OH group. The resonances at 61.5–66.0 ppm are
Scheme 1. Synthesis of APVA and CAPVA. (a) Reaction of PVA and acetaldehyde. (b) Cationization
of PVA.
Scheme 2. Meso and racemic rings of APVA.
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
Properties of a novel thermal sensitive polymer 337
assigned as unreacted PVAmethine carbon b. The methylene
carbons e and e0 of meso and racemic rings appear at
35.5–38.0 and 41.0–43.5 ppm, respectively. The resonances at
44.0–46.0 ppm are assigned as unreacted PVA methylene
carbon b. The methyl carbons d and d0 are observed at 21.1
and 21.3 ppm, respectively. The complicated unit sequences
of the APVA copolymer are responsible for the multiples.
The assignments of 13C-NMR spectra of CAPVA are shown
in Fig. 2b. The sharp and single peak at 53.5 ppm,
distinguished from other carbons, is the resonance of the
methyl carbon m on GTMAC side group, which is
contributed by three methyl groups. However, the reson-
ances of carbons i, l, and j are hidden in the signals of the
methylene and methine carbons of the backbone.
The DEPT spectrum of cationic PVA (CPVA) (Fig. 3) is
measured to assign the carbons i, l, and j in a simplified way
because the peaks of acetal rings are removed. The resonance
at 53.5 ppm is confirmed to be themethyl carbonm by the top
trace. Multiples at 44–47ppm in the second trace are the
methylene carbon a on the backbone of PVA, where the four
peaks are assigned to the following tacticities of PVA: rrr (at
46.2 ppm), rmrþmrr (at 45.8 ppm), mmrþmrm (at
45.4 ppm), and mmm (at 44.7 ppm).38,39 Two new and small
methylene signals were observed at 71.5 and 70.5 ppm in the
second trace also. They are reasonably assigned as the
methylene carbons i and j, respectively, since they are
directly attached to an oxygen or a nitrogen and shift farther
downfield than those methylene carbons with only methine
neighbor. The third trace had signals for methine resonance.
The peaks at 67.8, 65.8, and 63.8 ppm are assigned to be
methine carbon b in mm, mr, and rr triad sequences of PVA,
respectively. The two peaks at 64.7 and 64.5 ppm come from
the carbons j and h. The satisfied assignments of GTMAC
side group are reached by the DEPT spectrum.
Thermo-sensitivity of APVAPVA is a good water-soluble polymer at all temperature
rangeswhile the APVA shows thermo-sensitivity. To explore
Figure 1. 1H-NMR of APVA (a) and CAPVA (b) (Mw¼ 13,000–23,000).
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
338 H. Lu, A. Zheng and H. Xiao
the phase transition behavior and the thermal sensitivity of
the APVA, turbidity curves are used to characterize the LCST
of APVA aqueous solutions. The turbidity curves of starting
PVA and APVA are plotted in Fig. 4 as a function of
temperature. At low temperature, all of the three samples are
transparent solutions and their turbidities are close to 0,
indicating that the turbidity is independent of temperature.
However, after being heated to a certain temperature near
the LCST, the transparent solutions of APVA were observed
to become cloudy, followed by a drastic increase in turbidity.
The phase separation occurs undoubtedly and can be directly
viewed from the inset of Fig. 4. On the other hand, the
starting PVA solution remains clear and the turbidity is close
to 0 during heating. In our research, the LCST is defined as
the intersection of two straight lines drawn through the
curves of turbidity at low and high temperature, respect-
ively.
A series of APVAs with different chain length and degree
of acetalysis were prepared and their LCST transitions were
investigated since the LCST transitions of thermally sensitive
polymers depend on the molecular architecture, concen-
tration, and environmental conditions. Figure 5 shows the
LCST versus degree of acetalysis for three APVA with
different Mw. The LCST of APVA depends on the molecular
weight of the starting PVA and the degree of acetalysis.
Higher LCSTs were observed for the APVA with a shorter
chain length and lower degree of acetalysis. The LCSTs
decrease continuously with the increase in the acetalysis
Figure 2. 13C-NMR (DMSO-d6) of APVA (a) and CAPVA (b) (Mw¼ 13,000–23,000).
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
Properties of a novel thermal sensitive polymer 339
degree of APVA. At the same degree of acetalysis, the higher
the molecular weight of the starting PVA, the lower is the
phase transition temperature.
To visualize the changes in APVA chain conformation
during the phase transition, the atomic force microscope
(AFM) was used to image the actual physical states of
polymer chains at nanometer- or subnanometer-scale
resolution as a function of temperature. For AFM exper-
iments, a 0.1ml APVA (A28, Mw¼ 124,000, LCST is 33.88C)solution at 0.044 wt% was deposited on a freshly cleaned
silicon wafer and then dried at 20 or 608C. Figure 6a and b
show the representative AFM images of APVA films formed
Figure 4. Turbidity of a water solution containing 0.044 wt% PVA or APVA at
varying temperatures (Mw¼ 124,000–186,000).
Figure 3. The DEPT spectra of cationic PVA.
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
340 H. Lu, A. Zheng and H. Xiao
at 60 and 208C in air and observed under contact mode. AFM
images reveal that the process of association of APVA chains
occurs at 608C. The A28 film dried at 608C, above its LCST,
has a flattened globular appearance in the AFM image. Its
grains were about 2–5mm in diameter and 50–200 nm in
height. On the other hand, the film dried at 208C, below the
LCST of A28, had a smooth and uniform surface. This result
suggests that the interaction betweenA28 chains andwater is
strong for the APVA molecule in its fully extended state at
208C, i.e. A28 is completely dissolved in water and forms a
uniformly smooth film as the solvent is evaporated. At 608C,the A28 chains collapse and tend to aggregate to form A28
chain globules. The starting PVA films were prepared by the
same procedure. Both the PVA films, which dried at 20 and
608C, have quite smooth and uniform surfaces, similar to
those shown in Fig. 6b. (The AFM images are not presented
here.)
Thermal sensitivity of APVA is attributed to the hydro-
phobic acetal rings being introduced in the PVA chains. It is
assumed that, in water, the remaining hydrophilic hydroxyl
groups are in a good solvent and form intermolecular
hydrogen bonds with water, while the hydrophobic acetal
rings are in a poor solvent and form cage-like structures
surrounding them at low temperatures to make the APVA
dissolve in water. The free APVA chains remain disordered
with random coils in solution that are in full hydration,
which is particularly important for the stability of the APVA
chains in water. Heating of the solution causes destruction of
the hydrogen bonds and the exposure of acetal rings. The
hydrophobic groups tend to stick together in order to reduce,
as much as possible, their surface of contact with water,
leading to the formation of hydrophobic aggregates or
hydrophobic association. This is corresponding to a morpho-
logical change from theAPVA copolymer random coils to the
intermolecular hydrophobic association. As the temperature
increases, the number and size of hydrophobic aggregates
grow, which leads to strong light scattering within the
turbidimeter. As a result, a large increase in turbidity is
observed. The higher the degree of acetalysis of APVA, the
more hydrophobic the groups become, and as a result, a
lower LCST temperature.
The relatively inexpensive and large-scale commercial
polymer, PVA, is provided with the property of thermal
sensitivity by partial acetalization. Furthermore, the phase
transition temperature of the polymer can be readily
controlled by the degree of acetalysis. The natural properties
of PVA, mentioned in Introduction, and an adjustable LCST,
make the APVA a very promising material for a variety of
applications, such as sensor, switch, and recordingmaterials.
The acetal rings make the PVA thermally sensitive as well
as dissolvable in cold water. DSC was used to reveal the
Figure 6. AFM images of silicon wafers in A28 solution at (a)
608C and (b) 208C (0.044 wt%, Mw¼ 124,000).
Figure 5. Dependence of the phase transition temperature on the degree of acet-
alysis of APVA at the concentration of 0.044 wt%.
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
Properties of a novel thermal sensitive polymer 341
reasons why APVA is dissolved in water at room
temperature much more easily than PVA, which has a low
solubility at room temperature. Figure 7 shows the DSC
heating traces of PVA and APVA. As can be seen, the PVA
has only the crystal-melting peak (Tm) at 2158C, andA28 only
exhibits the glass transition temperature (Tg) at 988C. It
means that the starting PVA forms strong inter- and
intrachain hydrogen bonding and makes the polymer highly
crystalline. The Tg transition of PVA becomes very trivial
because the segments of PVA are frozen by the crystals. So
for the highly hydrolyzed PVA, a preparation temperature of
above 808C is required for complete dissolution in water in
an acceptable time to make sure that the inter- and
intrahydrogen bonding in the PVA chains are disrupted
by thermal energy. With the APVA, the acetal rings keep
APVA from forming crystals because they damage the orders
of molecular chain and decrease inter- and intrachain
hydrogen bonding. Consequently, APVA can be dissolved
in water at room temperature.
The effect of NaCl on the LCST behavior of(C)APVAAPVA is a linear nonionic polymer that is water-soluble at
low temperatures, but phase separates when the temperature
is raised above the LCST. Ionized APVA can also show
temperature sensitivity. Cationic charges are introduced
by grafting GTMAC onto the APVA chains in this study. But
the measurable LCST of CAPVA solution is reached only
when the polymer has a very small amount of cationic
groups and a large number of acetal rings or in NaCl
solution.
The effect of NaCl on the LCST of APVA and CAPVA
solutions is shown in Fig. 8. The addition of NaCl makes the
LCST of both polymers appear at a lower temperature. The
LCST of CAPVA is highly dependent on the NaCl
concentration at all experiment conditions. For C0.65A28,
the LCST decreases obviously even at a very low NaCl
concentration. On the contrary, the LCST of A28 remains
almost at the same temperaturewhen theNaCl concentration
is lower than 0.1M and the LCST begins to decrease quickly
after that. There is a critical point in the plot. The LCST of
C0.65A28 is higher than that of A28 before the critical NaCl
concentration, it is lower than that of A28 after that.
Figure 9 illustrates the dependence of LCST of three
CAPVAs, with the same amount of cationic groups but
different degrees of acetalysis, on the concentration of NaCl.
The LCSTs of three CAPVAs decrease at the same time with
the increase in NaCl concentration. However, CAPVA
containing a higher amount of cationic groups has a higher
LCST than others at all the experimental NaCl concen-
trations. If the amount of cationic group increases or the
degree of acetalysis decreases further, the LCST of CAPVA
will not be observed below 708C at a low NaCl concentration
and shifts to higher temperature at a high NaCl concen-
tration.
Figure 8. Dependence of LCSTof CAPVA and APVA on the concentration of NaCl
(Mw¼ 124,000–186,000, concentration of PVA¼ 0.044 wt%).
Figure 7. DSC heating traces of the starting PVA and APVA.
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
342 H. Lu, A. Zheng and H. Xiao
The cationic group and the NaCl concentration insure the
variation of the LCST. The cationic group increases the
expansion of CAPVA chains by the electrostatic repulsive
force in salt-free or low NaCl solution and the hydrophobic
aggregations are hindered greatly. The role of NaCl is not
only to screen the ionic interaction of CAPVA, but also to
alter the hydration of APVA andCAPVA. First, the Cl� anion
tends to have a stronger interaction with water molecules
and polarizes them. Thus, some of the original hydrogen
bonding between the hydroxyl of (C)APVA and water is
destroyed by the Cl�. Second, the Cl� ion interferes with the
hydrophobic hydration of the macromolecule by increasing
the surface tension of the cavity surrounding the backbone
and the acetal ring.40 These effects are similar to increasing
the temperature and lead to the salting-out of the polymers,
thereby lowering the LCST.
The reason why the LCST of CAPVA is lower than that of
APVA in high NaCl solution is discussed here. CAPVA has
more hydrophobic groups, introduced by the backbone,
acetal ring, and GTMAC, than the APVAwhen they have the
same degree of acetalysis. The ionic repulsion interaction
dominates the LCST in salt-free or low NaCl solution;
therefore, the LCST of CAPVA is much higher than that of
APVA. But the amount of hydrophobic group controls the
LCST of (C)APVA in high NaCl solution where the ionic
repulsive force is totally screened.
All of the above results mean that the LCST of (C)APVA
aqueous solution could bemodulated easily by themolecular
weight, degree of acetalysis, amount of cations, and NaCl.
LbL assembling of (APVA/CAPVA) on rayonfiberAPVA and CAPVA are thermo-sensitive water-soluble
polymers. Their thermal sensitivity makes them attractive
for applications that demand ‘‘smart’’ material responses to
environmental stimuli. The current work is attempted to
assemble the APVA and CAPVA multilayers on cellulose
fiber to render the fiber with temperature sensitivity. APVA
normally is a neutral polymer but shows small negative
charges in a basic solution, as shown in Table 1, because of
the residual hydroxyl group. The negative charges decrease
with the increasing of acetal rings. CAPVA shows positive
charges in a neutral aqueous solution and its charge density
remains invariant in solutions of different pH values.
Rayon fiber was chosen in this research as a model
cellulose substrate because its surface ismore even or smooth
compared to those from conventional mechanical or
chemical wood pulps. (A13/C0.7A11)8 and (A23/
C0.7A14)8 multilayers were assembled on rayon fiber in
0.44 wt% APVA or CAPVA solution with 5mM NaCl and
pH¼ 9 at room temperature. After assembling, rayon was
immersed into D-D water at 608C for 10min and dried in
oven at 608C to observe the thermal sensitivity of the surface.
Changes in surface morphology of (A13/C0.7A11)8 multi-
layers are revealed by the AFM images, as shown in Fig. 10.
Many more particles appear on the heat-treated surface. An
(A23/C0.7A14)8 assembled surface (AFM images not shown)
Figure 9. LCST versus NaCl concentration for 0.044 wt% CAPVA with molecular
weight 124,00–186,000.
Table 1. Charge density of APVA and CAPVA
APVAa CAPVAb
SamplesCharge
density (meq/g) SamplesCharges
density (meq/g)
A13 �0.073 C0.70 0.32A23 �0.056 C0.65 0.22A28 �0.043 C0.44 0.18A35 �0.039 C0.26 0.12A41 �0.036 C0.07 0.032
aMeasured with the solution of [APVA]¼ 10mM, [NaCl]¼ 5mMand pH¼ 9.bMeasured with the solution of [CAPVA]¼ 10mM, [NaCl]¼ 5mMand pH¼ 5–6.
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
Properties of a novel thermal sensitive polymer 343
presents the same thermal sensitivity. The roughness values
of heat-treated and heat-untreated surfaces listed in Table 2
further illustrate the behaviors, i.e. the roughness of the
heat-treated surface approximately doubled for rayon fiber
as substrate. In contrast, the roughness of control samples
without L-b-L assembly remained the same or similar.
The same assembling was also performed on a wafer as a
further proof for the thermally sensitive surface made from
APVA/CAPVA adsorption. TheAFM images and roughness
are presented in Fig. 11 and Table 2, respectively. The size
and the height of particles on the heat-treatedwafer aremuch
larger than those on the heat-untreated surfaces. The wafer
surface became rougher than the original one too.
We believe a surface-modified fiber can be obtained by the
multilayer assembly of APVA and CAPVA on fiber. CAPVA
was adsorbed on the rayon surface driven by electrostatic
interaction between the fiber and CAPVA. Then the APVA
and CAPVAwere alternately adsorbed onto the fiber to form
the multilayers.
CONCLUSIONS
We have shown here that novel thermally sensitive polymers
have been successfully synthesized by acetalyzing and/or
ionizing the commercially available PVA. The molecular
structures of acetalyzed PVA (APVA) and cationic APVA
(CAPVA) were characterized by NMR spectroscopy. The
effect of molecular weight, component and NaCl on the
phase transition of (C)APVA aqueous solution was inves-
tigated systematically. The phase transition is the result of
the introduction of the hydrophobic acetal rings onto the
PVA chains and can be changed easily.
(APVA/CAPVA)8 multilayers were assembled on rayon
fiber and silicon wafer to render the surfaces of substrates
temperature responsive. AFM images reveal that the
particles on the surface became larger after the material
was treated at 608C, and the roughness of the surfaces was
Figure 10. Flattened images of non heat-treated (a) and heat-treated (b) (A13/C0.7A11)8assembled multilayers on rayon fiber.
Figure 11. Flattened images of non heat-treated (a) and heat-treated (b) (A13/C0.7A11)8assembled multilayers on wafer.
Table 2. Roughness (Rq) of multilayers by AFM contact
mode (500 nm scale)
Fiber/nm Wafer/nm
Surface of substrate 208C 6.3 0.1608C 5.8 0.1
(A13/C0.7A11)8 208C 4.8 1.2608C 11.6 5.5
(A23/C0.7A14)8 208C 6.5 0.9608C 10.5 4.5
Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345
DOI: 10.1002/pat
344 H. Lu, A. Zheng and H. Xiao
also increased, thus providing direct evidence for the
thermo-sensitivity of the substrates assembled with
(APVA/CAPVA)8 multilayers.
The natural properties of PVA and the adjustable LCSTmake
(C)APVA to be a very promising material for a variety of
applications such as sensor, switch, and recording materials.
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